The hour of the wolf: Jennie Rooney unmasks the world of Cambridge spies

Boyd Tonkin interrogates the author about ideals, betrayals and regrets.

A sleuth in the past: Jennie Rooney (David Sandison)

In 1999, the 87-year-old "spy who came in from the Co-op" opened her front door in south London to face the waiting media. And she refused to say sorry. Melita Norwood had, over four decades after 1937, used her routine office job at the British Non-Ferrous Metals Association to pass classified information to the KGB and the previous Soviet agencies. Unmasked in files brought to Britain by defector Vasili Mitrokhin, the unrepentant "granny spy" was spared prosecution on the grounds of age.

Jennie Rooney, as a Cambridge University history student who dived into the turbid archives of 20th-century intelligence under the guidance of MI5's official chronicler Professor Chris Andrew, read Norwood's story. It struck a chord. "I just remember it so clearly: those pictures of her standing on the doorstop and saying - sort of - that she wasn't sorry. Basically, she was saying that I had my reasons and that I would do it again if those same circumstances were to arise."

Deliberately, Rooney has not fictionalised the Soviet heroine of the suburbs - who once went to Russia to collect a medal for her work - in Red Joan (Chatto & Windus, £12.99). In her third novel, the Liverpool-born writer (who still also works as a commercial solicitor) wanted to expose the secrets of the heart rather than mimic the data in the files. "The process of spying can be quite dull. It was more the reasons." Unlike Norwood, the committed life-long Communist, her protagonist Joan Stanley is wrenched by contradictory impulses, only briefly opts for betrayal, and then swiftly backtracks. "I wanted more ambiguity - for the character to be more torn."

In contrast to standard-issue espionage yarns in the Le Carré mould, with their frenzied and tortuous mole-hunts, Rooney has set the bar high for herself. Red Joan begins - in 2005 - with the former atomic scientist's confession, after the death of a fellow-agent from her Cambridge days. So we know she's "guilty" from page one: but of what precisely; and, above all, why?

"What drew me," Rooney explains as we talk in the top-floor bar of a West End hotel with a bird's-eye - or spook's eye - view of the city below, "was the actual story, not the genre of espionage fiction." She hopes that Red Joan "will be interesting to people who would normally say, 'I don't read thrillers.'" It will. But this "whydunnit" generates plenty of suspense as well. Joan, daughter of a progressive headmaster, and a science student at Newnham College, falls in 1937 into the orbit of the glamorous, cosmopolitan Russian-born cousins, Sonya and Leo. Shocked, despite her growing love for Leo, by his manipulative way with economic facts, Joan always keeps her distance from the "fairy-tale" of Soviet Communism. Even then, the comrades know that at its heart lurks a wolf named Stalin - but they prefer to hide his kills.

As war with Germany breaks out, and the Communist Party twists itself into theological knots to justify the Nazi-Soviet pact that held until Hitler's invasion of Russia in June 1941, Joan goes to do her patriotic duty as a research scientist in a Cambridge lab. Some readers may think the "Tube Alloys" project, which sees British researchers briefly take a lead in the conversion of the science of nuclear fission from theory to technology, a wild fiction. It all happened. Red Joan craftily braids a fictitious plot into the history of a secretive and little-known breakthrough. "They were really far ahead," says Rooney, "but once they get in with America it kind of got taken over. And the spies didn't help…" The Tube Alloys research was subsumed into the US-led Manhattan Project and then irrevocably tarnished by the betrayal of its discoveries at the hands of the "atom spy" Klaus Fuchs, exposed in 1949. Fuchs's own justifications helped Rooney to trace her heroine's own line of reasoning: "Some of the thoughts he had are ones that Joan has."

As collaboration with the Americans becomes tense and tangled, Joan resists all Leo and Sonya's inducements to treason. Then, in 1945, theory becomes practice and the atomic bombs fall on Hiroshima and Nagasaki. Now, with the West alone in command of a planet-smashing power, she concludes: "it's not about stealing; it's about sharing". Her career as a conduit for nuclear secrets begins. Norwood too believed that "Russia needed to be in with a chance." Red Joan digs deep into minds before hindsight. Rooney, who knows her way around the National Archives at Kew, has deployed her training to circumvent the historical novelist's usual lazy back-projection of present views onto past eras. "What I find interesting about history," she says, "is not what happened when but how did it feel - and what might I have done if I were in that position."

Rooney's parents - an accountant and a teacher - moved for a spell from Liverpool to Zambia, before the family relocated to south London. "It gives you a taste for other worlds," she reflects; "It was normal to me to have gone there." In an early introduction to the well-meaning smoke-and-mirrors of so many official sources, she remembers "being scared of snakes". So she asked her dad to walk her round the house to prove their absence. Later, he confessed, "We walked around and there were snakes - but you didn't see any of them!"

Her debut Inside the Whale - a shortlisted contender for the Costa first novel award - was written as she prepared for her law exams, and appeared in 2008. It drew, with warmth, humour and a cliché-free sense of period, on aspects of her family's wartime and post-war history, and revealed a notable talent for decade-leaping empathy in a genre too often marked by crude emotional anachronism. Its 2010 follow-up, The Opposite of Falling, moved further back in time to 19th-century experiments in flight, as an intrepid young woman travels to Niagara in an age pushed skywards by its dreams of freedom.

In Joan's case, those aspirations for a broader, better life - as a scientist, an independent woman, a citizen of the world - plausibly crash into the doubts and fears bred by a mainstream inter-war upbringing. When the crunch comes, she makes her choices on the battleground of conscience - although always with a divided soul. For Joan, after Hiroshima, "there are no sides any longer, not once this thing exists". And would her creator have taken a side? "It's impossible to tell," Rooney says. "As Melita Norwood said, in specific circumstances, at a particular time and given certain beliefs, I don't think that anyone can really say what they would have done for sure."

Rooney never intended Red Joan as a parable of today's dilemmas over commitment and betrayal, while the Soviet Union itself - whether as beacon or as bogey-man - fell into history in 1991. But she does acknowledge that the core plot may still resonate. There is, she notes, "a parallel with terrorism. Until you understand why it is that people do what they do, you can't really get around to solving it."

So before history's hindsight kicks in, Joan wrestles with her mixed motives as the "wolf" of Moscow seems to stand in need of protection from an earth-shattering force. "At times I thought, if I hadn't framed it like this, maybe it would have been easier," Rooney says. "But that was always the picture I had in my head: of Melita Norwood standing in her garden with a piece of paper, saying, 'Yes. I did it'."

Jennie Rooney will be appearing at the 'Independent' Bath Literature Festival on 9 March: www.bathlitfest.org.uk